The National Institute of Environmental Research under the Ministry of Climate, Energy and Environment said on the 7th that ultrafine dust was at a "bad" level in Seoul, Incheon, southern Gyeonggi, the Chungcheong region, and North Jeolla. The "bad" level of ultrafine dust started in Sejong on the night of the 5th and expanded on the 6th to Seoul, Incheon, southern Gyeonggi, the Chungcheong region, Gwangju Metropolitan City, and North Jeolla. Before that, all regions were at "moderate" or "good."
Experts said this was due to a seasonal factor called an "inversion layer phenomenon." Usually, convection continuously occurs as cold air descends and warm air rises, circulating the air.
But between fall and winter, the movement of air changes. This is because radiative cooling occurs at night, when the heat of the ground, warmed by sunlight during the day, is released in large amounts into outer layers of the atmosphere. When this happens, cold air stays below and warm air stays above, and an "inversion layer" forms in which circulation does not occur properly.
An official at the National Institute of Environmental Research said, "When an inversion layer occurs, the air does not move up and down and is placed in a condition where it is difficult to mix," and added, "As if a lid were put on, cold air gets trapped near the surface, and, along with it, pollutants in the air are not well dispersed."
Poor fine dust conditions are expected to appear from time to time until spring next year. An official at the National Institute of Environmental Research said, "If fine dust increases due to the inversion layer phenomenon in spring and late fall, in winter there is a lot of fine dust coming in from overseas," and added, "From November, fine dust begins to increase and the air quality tends to remain poor until March of the following year."